subdivided/unsubdivided linked split screen
A trick to do that :



Temporary use a mirror (1) with another object as center, here an empty (2).

You can edit the original and see the mirrored mesh with subsurf.

When finished, remove or hide the mirror modifier.

Just remember that the shape is mirrored... so the left becomes the right, etc.

From that you can set two views in order to choose the rendering mode for each: